VHP Photo Glossary: Pumice

Photograph by W.E. Scott on 27 June 1991

Pumice

Pumice is a light, porous volcanic rock that forms during explosive
eruptions. It resembles a sponge because it consists of a network of
gas bubbles frozen amidst fragile volcanic glass and minerals. All types
of magma (basalt, andesite, dacite, and rhyolite) will form pumice.

Pumice is similar to the liquid foam generated when a bottle of pressurized
soda is opened--the opening depressurizes the soda and enables
dissolved carbon dioxide gas to escape or erupt through the opening.
During an explosive eruption, volcanic gases dissolved in the
liquid portion of magma also expand rapidly to create a foam or
froth; in the case of pumice, the liquid part of the froth quickly
solidifies to glass around the gas bubbles.